Is there a fault, and does the diagnosis hold within laboratory error?

1. Choose the insulating fluid: mineral oil, natural ester (soybean, rapeseed, sunflower or unspecified), synthetic ester or silicone. For mineral oil, say whether the sample comes from the main tank, a main tank with a communicating on-load tap changer, or the tap changer compartment itself.
2. Enter the gases in ppm from your laboratory report: H2, CH4, C2H6, C2H4 and C2H2 are needed for a diagnosis; CO, CO2, O2 and N2 are optional. Add earlier dated samples to get the rate of change, or paste a row copied from a spreadsheet.
3. Read the summary: severity, fault type from each method, the reasons to treat the result with caution, and the recommended action. The detail shows every Duval triangle and pentagon with your sample plotted.

An empty field means "not measured" and is never read as zero. With one sample and no date, the calculator uses today's date; with several samples, each needs its date.

Mineral oil, main tank. Severity follows the status of IEEE C57.104-2019 §6.1.1 and its Tables 1 to 4 (gas levels by age and O2/N2 ratio, increments and rates). IEC 60599:2022 adds the typical values of Annex A (Tables A.2 and A.3), the gas ratios of Table 1 and the CO2/CO ratio of §5.5, shown as indicative. Fault type comes from Duval Triangle 1 with the 2022 arcing sub-zones in oil (-H) and in paper (-P), Duval Triangles 4 and 5 as activated in CIGRE TB 771 (2019) Appendix H.1, and Duval Pentagons 1 and 2. The oil-and-paper sub-zones are indicative: the tool says so next to the result and suggests confirming arcing in paper with other evidence, such as acoustic tests and the CO2/CO ratio.

Tap changer compartment. The main-tank tables do not apply. The calculator plots Duval Triangle 2 (IEEE C57.139 Table D.2) and gives no diagnosis when C2H2 and C2H4 are both below 10 ppm (CIGRE TB 443 (2010) §4.5). For the full IEEE C57.139-2015 check of a tap changer, with ratios, trend and gas per operation, use the OLTC DGA Check.

Natural and synthetic esters. Typical values from IEEE C57.155-2014 Table 1 and IEC 63585:2026 Annex B. Fault type from Duval Triangle 3, or 3b when the solid insulation is cellulose, Triangles 6 and 7 for soybean ester, and Pentagons 3, 4 and 4b. Rapeseed and unspecified esters use the soybean methods, marked as indicative. Mineral-oil methods are never applied to an ester.

Silicone. Only the silicone version of Duval Triangle 3; severity is not assessed.

Severity rules. IEC values above the typical range give severity 2, never 3, because the only action IEC 60599:2022 attaches to them is more frequent sampling. Esters reach severity 2 at most, following IEEE C57.155-2014 §6.2.

Agreement between methods. The calculator groups each method's zone into fault families (partial discharge, discharges, mixed, thermal, stray gassing) and reports whether the methods agree, are compatible or differ.

A Duval point near a zone boundary can change zone when a gas moves by a few ppm, and laboratory results are never exact. The calculator varies each gas across its analytical uncertainty and counts how much of that range keeps the same zone. It uses ±15 % above 10 ppm and ±30 % from 5 to 10 ppm, derived from the uncertainty figures of IEC 60599:2022 §6.2, and ±75 % below 5 ppm, following CIGRE TB 783 (2019) §5.3. For a triangle this is a grid of 9 levels per gas; for a pentagon, 5 levels on each of the five gases.

The zone is reported as stable only when it stays the same over the whole grid. Otherwise the result is borderline and the page lists the other zones with their share. The calculator does not turn this into a confidence percentage: it lists the reasons for caution (low or weak concentrations, borderline zone, methods that differ, single sample), each with its source.

A high-temperature thermal fault, and a zone on a boundary

Mineral oil, main tank, transformer age 22 years, one sample: H2 150, CH4 260, C2H6 80, C2H4 520, C2H2 9, CO 250, CO2 2500, O2 2200 and N2 53000 ppm. This is the "T3" example built into the calculator.

• Duval Triangle 1: T3, thermal fault above 700 °C.
• Duval Pentagon 2: T3-H, thermal fault in oil only.
• The Triangle 1 zone is stable: it stays T3 across the whole uncertainty grid.
• The only reason for caution listed is that there is a single sample.

A second example sits on a boundary. With H2 50, CH4 80, C2H6 30, C2H4 20 and C2H2 0 ppm, Triangle 1 falls at 20 % C2H4, on the line between T1 and T2. The calculator marks the zone as borderline and names the alternative, so you know that one more sample, or a laboratory check, matters more than the label.

A third built-in example, with H2 160, CH4 20, C2H6 4, C2H4 14 and C2H2 60 ppm, gives Triangle 1 zone D1, discharges of low energy.

The T3 and D1 cases are in the tool's examples menu; the boundary case can be typed in.

A result such as "<0.5" is not zero: it only says the gas is somewhere between zero and 0.5 ppm. The calculator keeps the limit, runs each method at both ends, 0 and the limit, and gives a result only when both ends agree. When they disagree, that method abstains and a note names the method and the gas, following IEEE C57.104-2019 §5.2.1 on the large relative uncertainty near the detection limit. "ND" without a number stays unknown, and the page asks for the limit.

For example, C2H2 reported as "<0,5" with C2H4 at 2.0 ppm gives a C2H2/C2H4 ratio between 0 and 0.25, shown as a range, not as a single value.

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